Magnetic interrogation techniques

ABSTRACT

A reader for interrogating a magnetic tag, e.g. for reading data stored in the tag, is described. The reader comprises a field generating device the magnetic field produced by which defines an interrogation zone, wherein said field generating device comprises: (a) means for generating a magnetic field: (b) a transmit coil for transmitting an interrogating electromagnetic signal into the interrogation zone so as to interact with a magnetic tag, when present in said interrogation zone; and (c) at least one receive coil for receiving an electromagnetic signal generated by a tag in response to said interrogating signal and the magnetic field produced by said magnetic field generating means. The field generating means can take the form of a pair of hollow cylinders, giving rise to a “loop” reader, or it can be a flat magnet, giving rise to a “side pass” reader.

In previous patent applications, in particular PCT/GB96/00823 (WO96/31790) and PCT/GB96/00367 (WO 97/04338), we have described andclaimed novel techniques for spatial magnetic interrogation and noveltags. The technology described in WO 96/31790 is based on exploiting thebehaviour of magnetic materials as they pass through a region of spacecontaining a magnetic null. In particular, these earlier applicationsdescribe, inter alia, how passive tags containing one or more magneticelements can perform as remotely-readable data carriers, the number andspatial arrangement of the elements representing information.

in the above applications we described a number of possible systemembodiments employing either permanent magnets or electromagnets tocreate the magnetic null. We also described several systemimplementations some of which are particularly appropriate for tagsemploying very low coercivity, high permeability magnetic elements.These implementations work by detecting harmonics of a superimposedlow-amplitude alternating interrogation field.

In a later application, GB9612831.9, and its successor PCT/GB97/01662,we describe arrangements which work by detecting the baseband signalsgenerated by the passage of the tag through the magnetic null, withoutthe need for any superimposed alternating interrogation field. Aspecific design for a reader in the form of a narrow slot was describedin GB9612831.9.

The present application relates to magnetic readers which can be used toread data from magnetic tags operating on the principles described in WO96/31790 and/or in WO 97/04338.

According to on aspect of the present invention, there is povided areader for interrogating a magnetic tag, e.g. for reading data stored inthe tag, which reader comprises a field generating device the magneticfield produced by which defines an interrogation zone, wherein saidfield generating device comprises (a) means for generating a magneticfield; (b) a transmit coil for transmitting an interrogatingelectromagnetic signal into the interrogation zone so as to interactwith a magnetic tag, when present in said interrogation zone; and (c) atleast one receive coil for receiving an electromagnetic signal generatedby a tag in response to said interrogating signal and the magnetic fieldproduced by said magnetic field generating means.

In one embodiment, the field generating device is in the form of a pairof concentric hollow cylinders; with this embodiment, the interrogationzone is defined by the interior of the inner of the two concentriccylinders.

In another embodiment, the field generating device is a flat relativelythin magnet which has poles of one polarity, e.g. north, on one face andpoles of the opposite polarity, e.g. south, on the other face. With thisembodiment, the interrogation zone is defined by the volume of spacedisposed immediately adjacent to one of the faces of the magnet, theextent of the volume in the direction perpendicular to the plane of themagnet being governed by the extent of effective interaction betweenthemagnetic field and a tag. This second embodiment is, in effect, asingle-sided reader—i.e. a device for reading a magnetic tag whichoperates when the tag passes across the face of the reader.

These two embodiments will now be described in greater detail.

FIRST EMBODIMENT

The concentric hollow cylinders are preferably squat—i.e. the diameterof the device is relatively large compared to its length.

Preferably each of the concentric hollow cylinders supports or containsthe means for generating a magnetic field; this is preferably a sourceof permanent magnetism, e.g. a ferrite magnet. The magnetic fieldsgenerated by such an arrangement is preferably radial—i.e the innersurface of a given one of the cylinders carries magnetic poles of afirst polarity, e.g. north, while the outer surface of that cylindercarries magnetic poles of the opposite polarity, e.g. south. Further, itis preferred for the magnetic poles on the inner surface of the outercylinder to correspond to those on the outer surface of the innercylinder—opposed surfaces carrying poles of the same polarity—so thatthe arrangement may, for example, be S-N:N-S.

The presently preferred arrangement is to employ cylinders formed of aflexible polymer impregnated with a ferromagnetic material, e.g.ferrite. A suitable material for this purpose is “FLEXOR 15”manufactured and sold by Eclipse Magnetics Limited of Sheffield,England.

Preferably each of the two hollow cylinders carries both transmit andreceive coils. The transmit coils are preferably wound on the cylindersin the same sense; and the receive coils are preferably wound on the twocylinders in opposite senses and connected in series.

The coils (transmit and receive) can be wound on the inner or outersurfaces of the two cylinders.

The transmit (Tx) coils are preferably positioned so that the netcoupling between transmit and receive (Rx) coils is zero. This isadvantageous in that it permits the use of relatively simple electronicsto process the output from the receive coils and renders the system lesssensitive to distortions in the transmitted waveform.

The transmit coils (one on each of the two cylinders) are generallyconnected in series. The windings of the inner transmit coil arepreferably restricted to two zones—i.e. are clumped together on twobundles, rather than being uniformly or quasi-uniformly spread out overthe surface which carries them. This arrangement gives a more uniformtransmitted field across the radius of the interrogation zone.

The receive coil(s) can be wound uniformly or quasi-uniformly over thesurface(s) which carry(ies) them.

The contrary-winding arrangement for the two receive coils describedabove is advantageous because it permits an overall Rx coil arrangementwhich has zero net dipole moment, the opposed dipole contributions ofthe two component Rx coils cancelling out. This in turn gives theoverall Rx coil arrangement an octopole type fall off (i.e. proportionalto r⁻⁵) sensitivity with distance away from the coil (as compared to adipole type fall off, which is proportional to r⁻³), thereby resultingin very good rejection of magnetic interference from extraneous sources(e.g. VDU screens).

SECOND EMBODIMENT

Preferably the flat relatively thin magnet employed in this embodimentis in the form of a polymeric material containing permanently magnetisedparticles, e.g. of ferrite. For example, the material “FLEXOR 15”manufactured and sold by Eclipse Magnetics Limited of Sheffield, Englandmay be used to form a thin (e.g. 0.5 mm) permanent magnet. Preferablythe device includes two receive coils placed on either side of thetransmit coil, all three coils being adjacent to, but spaced from, oneface of a permanent magnet such as that just described. With such anarrangement, it is advantageous for the axes of the receive coils to bemutually parallel, and—to be perpendicular to the axis of the transmitcoil. This arrangement minimises direct coupling between the transmitand receive coils. For example, the axes of the receive coils may besubstantially parallel with the magnetic filed direction produced by thepermanent magnet; and the axis of the transmit coil perpendicular to themagnetic field direction. This geometry is preferred because thesensitivity of the receive coils then has a quadrupole characteristic,thus allowing the reader to be relatively insensitive to interferencecaused by extraneous magnetic sources, e.g. VDU screens.

Where two receive coils are used, they are preferably wound in oppositesenses, and connected electrically in series. This arrangement resultsin a zero dipole moment.

A reader in accordance with this second embodiment of the invention canhave relatively small dimensions—e.g. 25 mm×25 mm×10 mm; and can “read”data from a magnetic tag without requiring direct contact between thetag and the reader. Typically, the reader is able to function adequatelyover a range of several millimetres—e.g. with a spacing of 3-5 mmbetween the tag and the reader.

Readers in accordance with this embodiment are of particular use inreading data from a series of tags presented sequentially to the reader;this is of benefit, for example, in apparatus which dispenses articlescarrying a magnetic label or tag for identification/stockcontrol/pricing or other commercial purposes. One example of suchapparatus is a vending machine.

ILLUSTRATIONS OF THE INVENTION

The invention will now be described, by way of example, with referenceto the accompanying drawings, in which:

FIGS. 1a and 1 b illustrate the winding of the transmit (Tx) and receive(Rx) coils in one embodiment of “loop readers” in accordance with thisinvention;

FIGS. 2a and 2 b show schematically the arrangement of the magneticfield producing parts of the reader of FIG. 1;

FIG. 3 shows schematically how the radial magnetic field (H_(r)) and thefield gradient dH_(Z)/d_(z) varies with radial distance (r) from thedetection plane in the reader of FIG. 1;

FIG. 4 shows a currently preferred circuit design for use with thereader of FIGS. 1 and 2;

FIG. 5 illustrates a typical output signal from a reader in accordancewith this embodiment of the invention;

FIG. 6 shows a plan view of part of a drinks vending machine whichincludes a “side pass reader” in accordance with the second embodimentof the invention; and

FIG. 7 shows a front elevational view of the reader FIG. 6.

Referring now to the drawings, FIGS. 1(a) and 1(b) show the arrangementof Tx and Rx coils in this embodiment. The coils are supported by thehollow cylindrical magnetic bodies (not shown in FIG. 1—see FIG. 2).FIG. 1a is a cross-section perpendicular to the axes of the coils; andFIG. 1b is a plan view of the coils. As shown, the outer receive coil R₁comprises 68 turns of radius 109 mm; and inner receive coil R₂ comprises130 turns of radius 78 mm. The windings of R₁ and r₂ are opposite insense, as indicated (using the dots and crosses convention); these tworeceive coils are connected in series.

Outer transmit coil T₁ comprises 110 turns of radius 109 mm and innertransmit coil T₂ comprises 40 turns of radius 79 mm. Both transmit coilsare wound in the same sense, and are connected in series.

This arrangement of the coils results in zero net coupling between Rxand Tx coils, and ensures that the Rx coils have zero net dipole moment.

Referring next to FIG. 2, the two hollow cylindrical bodies 1 and 2 areformed of “FLEXOR 15” and are disposed so that outer body 1 has southmagnetic poles on its on its outer surface and north magnetic poles onits inner surface; while inner body 2 has north magnetic poles on itsouter surface and south magnetic poles on its inner surface. The centralspace 3 defined within body 2 constitutes the interrogation zone throughwhich, in use, a magnetic tag is passed. The plane 4 perpendicular tothe axes of the coaxial cylinders 1 and 2 defines a detection plane.Magnetic body 1 has a thickness of 5 mm and an axial extent of 50 mm;and magnetic body 2 has a thickness of 1 mm and an axial extent of 40mm. The field pattern generated by this arrangement has a substantiallyuniform gradient over the whole of detection plane 4, and hence theminimum radial field within the detection plane itself. This permits thefield gradient to be relatively high (typically 3-5 kA/m), therebygiving better resolution, without causing a magnetic tag (e.g. anIST-type thin film material with anisotropic magnetic properties) whosedata is being read to become saturated by the magnetic field along itsnon-preferred axis of magnetisation.

FIG. 3 shows how the field gradient dH_(Z)/d_(Z) is substantiallyconstant across the interrogation zone (i.e. practically invariant withr); and also that the radial field H_(r) is essentially zero at thedetection plane, and increases rapidly with radial distance from thatplane. The dashed lines show how the two plots would be if the innerbody 2 were removed, thus demonstrating the effectiveness of thisinvention.

FIG. 4 illustrates a preferred electrical circuit for a reader inaccordance with this invention. The circuit comprises a sine wave drive10 operating at 12.5 kHz; a frequency divider (÷2) 11; a low pass filter12 removing frequencies significantly higher than 6.25 kHz; a bufferamplifier 13; and a capacitor 14 of 150 nF connected to one end of thetransmit coils 15; these have an inductance of 4 mH and carry a currentof 0.1 A rms. This gives a value of H_(z) at the centre of the “loop”defined by the cylinders 1, 2 of approximately 65 A/m rms.

On the receive coils (16) side of the circuit, a capacitor 17 is placedin parallel with the coils 16, and the output is passed to a band passfilter 18 operating at 12.5 kHz±1 kHz. The filtered output is suppliedto a buffer amplifier 19 and is then combined at 20 with the sourcesignal from driver 10. The summed signals then pass to a 1 kHz low passfilter 21 whose output 22 represents the output of the reader.

A typical output from the reader is illustrated at FIG. 5; the detailedsignal depends on the magnetic characteristics of the tag which is beinginterrogated.

Referring nest to FIG. 6 of the drawings, the reader 31 is positionedadjacent to a carousel 32 which conveys drinks cartons such as 33 a, 33b and 33 c. Each of these carries a magnetic label or tag 34 which istypically an IST material—a thin film anisotropic magnetic material wellknown in connection with anti-pilferage applications.

The reader 31 comprises a permanent magnet 35, a first receive coil 36,a second receive coil 37 and a transmit coil 38. The coils are spacedfrom magnet 35 by a distance of about 5 mm. Receive (Rx) coils 36 and 37are wound in antiphase and are connected in series. The axes of the RXcoils 36, 37 are parallel with the field direction while the axis of thetransmit (TX) coil 38 is perpendicular to the field direction.

The dimensions of the reader in this embodiment are of the order of 25mm×25 mm×10 mm. Each of the coils comprises 100 turns of 0.1 mm copperwire. The output of the Rx coils forms part of an electrical circuitsuch as that described above in conjunction with FIG. 4.

In operation, carousel 32 turns in the direction of arrow 39 as drinksare ordered and dispensed. As each container 33 moves round thecarousel, its magnetic label 34 is oriented outwardly so as to be readby the reader device 31. Such a label may typically comprise a series ofactive magnetic regions of about 3 mm×3 mm separated by non-magneticregions or gaps typically about 1 mm (seen in the direction of movementof the container).

Typically, each of the coils may comprise 100 turns of 0.1 mm copperwire.

The invention is not restricted to the embodiments described above,which are given as non-limiting examples.

What is claimed is:
 1. A reader for interrogating a magnetic tag havingat least one magnetically active element, which reader comprises: (1) afield generating means for generating a magnetic field within aninterrogation zone; and (2) at least one receive coil for receiving anelectromagnetic signal generated by the magnetically active element(s)in response to the field generated by said magnetic field generatingmeans, wherein the field generating means comprises: (a) at least onepermanent magnet to create a static magnetic field within saidinterrogation zone; and (b) at least one transmit coil for generating analternating magnetic field within said interrogation zone.
 2. A readeras claimed in claim 1, characterized in that said permanent magnet(s) isin the form of a pair of concentric hollow cylinders, the interrogationzone being defined by the interior of the inner of the two concentriccylinders.
 3. A reader as claimed in claim 2, wherein said concentrichollow cylinders are squat—i.e. their diameter is relatively largecompared to their length.
 4. A reader as claimed in claim 2, whereineach of the concentric hollow cylinders supports or comprises apermanent magnet arranged to generate a radial magnetic field, whereinthe inner surface of one of the cylinders is of a first magneticpolarity, while the outer surface of that cylinder is of the oppositepolarity.
 5. A reader as claimed in claim 4, wherein the magnetic poleson the inner surface of the outer cylinder correspond to the magneticpoles on the outer surface of the inner cylinder, so that opposedsurfaces carry magnetic poles of the same polarity.
 6. A reader asclaimed in claim 2, wherein said cylinders are each formed of a flexiblepolymer impregnated with a ferromagnetic material.
 7. A reader asclaimed in claim 6, wherein the ferromagnetic material is ferrite.
 8. Areader as claimed in claim 2, wherein each of the hollow cylinderscarries both transmit and receive coils.
 9. A reader as claimed in claim2, wherein the transmit coils are wound on the two cylinders in the samesense; and the receive coils are wound on the two cylinders in oppositesenses and are connected in series.
 10. A reader as claimed in claim 1,characterized in that said permanent magnet is in the form of a flatrelatively thin magnet in which one face is of a first magnetic polarityand the other face is of the opposite polarity, wherein theinterrogation zone is disposed immediately adjacent to one of the facesof said magnet.
 11. A reader as claimed in claim 8, which comprises tworeceive coils placed on either side of a single transmit coil, all threecoils being adjacent to, but spaced from, one face of said permanentmagnet.
 12. A reader as claimed in claim 11, wherein the axes of thereceive coils are mutually parallel, and are perpendicular to the axisof the transmit coil.
 13. A reader as claimed in claim 12, wherein theaxes of the receive coils are substantially parallel to the magneticfiled direction produced by the permanent magnet; and the axis of thetransmit coil is substantially perpendicular to said magnetic fielddirection.
 14. A reader as claimed in claim 11, wherein said receivecoils are wound in opposite senses (i.e. in antiphase), and areconnected electrically in series.
 15. A reader as claimed in claim 8wherein the flat relatively thin magnet is in the form of a polymericmaterial containing permanently magnetized particles.
 16. A reader asclaimed in claim 15, wherein the flat relatively thin magnet is in theform of a ferromagnetic material including ferrite.
 17. A reader forinterrogating a magnetic tag having at least one magnetically activeelement, which reader comprises: (1) a field generator for generating amagnetic field within an interrogation zone; and (2) at least onereceive coil for receiving an electromagnetic signal generated by themagnetically active element(s) in response to the field generated bysaid magnetic field generator, wherein the field generator comprises:(a) at least one magnet for creating a static magnetic field within saidinterrogation zone; and (b) at least one transmit coil for generating analternating magnetic field within said interrogation zone.
 18. A readeras defined in claim 17 wherein said magnet for creating a staticmagnetic field is a permanent magnet.
 19. A reader for interrogating amagnetic tag having at least one magnetically active element, whichreader comprises: (1) a field generating means for generating a magneticfield within an interrogation zone; and (2) at least one receive coilfor receiving an electromagnetic signal generated by the at least onemagnetically active element in response to the field generated by saidmagnetic field generating means, wherein the field generating meanscomprises: (a) a means to create a static magnetic field within saidinterrogation zone, wherein said static magnetic field comprises a firstregion at which the component of the magnetic field resolved in a firstdirection is zero, and where in regions contiguous with said firstregion the component of the magnetic field resolved in said firstdirection is sufficient to saturate the, or a part of the, at least onemagnetically active element; and (b) at least one transmit coil forgenerating an alternating magnetic field within said interrogation zone.20. A reader for interrogating a magnetic tag having at least onemagnetically active element, which reader comprises: (1) a fieldgenerator for generating a magnetic field within an interrogation zone;and (2) at least one receive coil for receiving an electromagneticsignal generated by the magnetically active element in response to thefield generated by said magnetic field generator, wherein the fieldgenerator comprises: (a) a means to create a static magnetic fieldwithin said interrogation zone, wherein said static magnetic fieldcomprises a first region at which the component of the magnetic fieldresolved in a first direction is zero, and where in regions contiguouswith said first region the component of the magnetic field resolved insaid first direction is sufficient to saturate the, or a part of the, atleast one magnetically active element; and (b) at least one transmitcoil for generating an alternating magnetic field within saidinterrogation zone.